Along with the microfabrication of a semiconductor device, processing accuracy required for an etching process of a wafer has been increasingly strict year after year. In order to perform a highly accurate process for finely patterning a wafer surface with a plasma processing apparatus, it is important to control the temperature of the wafer surface during etching processes. However, due to the demand for a larger area of a wafer and the improvement of etching rate, high frequency electric power applied to the plasma processing apparatus tends to be increased, and large electric power in kilowatt-order has begun to be applied, in particular, for etching insulating films. The application of large electric power increases ion impact energy to the wafer surface, which involves a problem of an excessive rise in the temperature of the wafer during the etching process. Moreover, due to the demand for further improvement of processed shape accuracy, a means capable of controlling the temperature of the wafer at high speeds and providing in-plane temperature uniformity during the process has been required.
In order to control the surface temperature of the wafer in a plasma processing apparatus, it is necessary to control the surface temperature of a wafer stage (having an electrostatic chuck) which comes in contact with the back surface of the wafer through a heat transfer medium. The control of the surface temperature of a conventional wafer stage is exercised by feeding a liquid refrigerant through a passage formed inside the wafer stage. The liquid refrigerant is first adjusted to a target temperature by a cooling unit or a heating unit inside a refrigerant supply device and supplied to the passage inside the wafer stage. Such a refrigerant supply device is designed to once store the liquid refrigerant in a tank and feed the refrigerant after adjusting its temperature, and is effective in keeping the surface temperature of the wafer constant because the liquid refrigerant itself exhibits a large heat capacity. However, the refrigerant supply device is poor in temperature response, low in heat exchange efficiency, and has difficulty in achieving agile temperature control. Therefore, a recent high heat input has made the refrigerant supply device larger in size and has created a difficulty for the device in optimally controlling the temperature of the wafer surface in accordance with the progression of etching.
In view of the circumstances described above, there has been proposed a direct-expansion type refrigerant supply device (hereinafter, referred to as “a direct-expansion cooling system”) in Japanese Patent Application Laid-Open Publication No. 2008-034409 in which a refrigerant circulation system evaporates a refrigerant in a refrigerant passage of the wafer stage to cool the wafer stage with a compressor for applying a high pressure to the refrigerant, a condenser for condensing the high-pressure refrigerant, and an expansion valve for expanding the refrigerant. The direct-expansion cooling system that uses latent heat generated by refrigerant evaporation exhibits high cooling efficiency and enables agile control of the refrigerant's evaporation temperature by pressure. For the reasons described above, adoption of the direct-expansion cooling system as a device for supplying a refrigerant to a wafer stage realizes highly-efficient high-speed temperature control of a semiconductor wafer during a high heat input etching process.
In addition, Japanese Patent Application Laid-Open Publication No. 2008-187063 discloses a method for determining whether a refrigerant in a refrigerant passage of a wafer stage has been completely evaporated by installing a thermometer for the refrigerant at a refrigerant inlet port and a refrigerant outlet port of the wafer stage.
Furthermore, Japanese Patent Application Laid-Open Publication No. 2008-186856 discloses a method for preventing a refrigerant in a refrigerant passage from drying out (vanish of liquid films) by controlling the degree of dryness of the refrigerant discharged from the wafer stage in order to uniformly cool the surface of the wafer stage with the direct-expansion cooling system.